The present invention relates to a liquid crystal display, and more particularly to a twisted nematic liquid crystal display showing a high contrast and a wide angle of visibility. The present invention also relates to a method for fabricating a liquid crystal display as well as a method for driving the same.
The conventional liquid crystal display has a relatively narrow angle of visibility due to a unique action of the liquid crystal. The reason for the narrow angle of visibility will be described with reference to a twisted nematic liquid crystal display driven by thin film transistors. FIG. 1 illustrates twisted nematic liquid crystal molecules between two glass substrates when no voltage is applied to the liquid crystal. FIG. 2 illustrates the twisted nematic liquid crystal molecules between the two glass substrates when a voltage for driving the display is applied to the liquid crystal.
As illustrated in FIGS. 1 and 2, liquid crystal molecules 11 have a slender shape. A longitudinal direction of the liquid crystal molecule may be referred to as a direction of the liquid crystal molecule. When no voltage is applied to the liquid crystal, the liquid crystal molecules 11 adjacent to the substrates 23 and 33 are pretilted by a pretilted angle 13 from the surfaces of the substrates 23 and 33. Since the pretilted angle 13 is small, the liquid crystal molecules 11 are aligned nearly parallel to the surfaces of the substrates 23 and 33.
A horizontal direction of the liquid crystal molecule 11 may be referred to as an orientation of the liquid crystal molecule 11. The liquid crystal molecules 11 aligned between the substrates 23 and 33 are twisted around an axis perpendicular to the surfaces of the substrates 23 and 33 so that the liquid crystal molecule 11 on the surface of the substrate 33 differs in orientation by 90 degrees from the liquid crystal molecule 11 on the surface of the substrate 23. Such twist of the liquid crystal molecules 11 aligned between the substrates 23 and 33 is not illustrated in FIGS. 1 and 2.
There is no remarkable dependency of the visibility upon the direction of view because the liquid crystal molecules 11 are aligned nearly parallel to the surfaces of the substrates 23 and 33.
When a voltage is applied to the liquid crystal molecules 11 aligned between the substrates 23 and 33, then the directions of the liquid crystal molecules 11 are changed as illustrated in FIG. 2. The directions of the liquid crystal molecules 11 apart from the surfaces of the substrates 23 and 33 are largely changed to approach the vertical direction, whilst the direction of the liquid crystal molecules 11 adjacent to the surfaces of the substrates 23 and 33 almost remains unchanged from the pretilted angle 13. In FIG. 2, the liquid crystal molecules 11 are directed to "right-up". As the liquid crystal molecules 11 approach to the vertical direction, a transmissivity of a ray of light is raised. This means that when a voltage is applied to the liquid crystal, the transmissivity of a ray of light is raised whereby a color of the display approaches dark or black, assuming that a display is operational in a normally-white mode. By contrasts, as illustrated in FIG. 1, if no voltage is applied to the liquid crystal then the transmissivity of a ray of light is dropped and a probability of scattering of light by the parallel liquid crystal molecules 11 is raised. Those result in that a color of the display approaches white.
During no voltage application, the directions of the liquid crystal molecules 11 remain relatively parallel to the surfaces of the substrates 23 and 33 as illustrated in FIG. 1. Notwithstanding, during the voltage application, the liquid crystal molecules 11 have large tilted angles from the surfaces of the substrates 23 and 33 and approach to the "right-up" as illustrated in FIG. 2. In the latter case, a ray of light 41 is directed nearly parallel to the largely tilted directions of the liquid crystal molecules 11 apart from the surfaces of the substrates 23 and 33, whilst a ray of light 42 is, however, directed nearly vertical to the largely tilted directions of the liquid crystal molecules 11 apart from the surfaces of the substrates 23 and 33. Needless to say, a transmissivity of the ray of light 41 is much higher than a transmissivity of the ray of light 42. This means that if a direction of view is parallel to the direction of the ray of light 41 then the color of display approaches to the dark or black. If, however, a direction of view is parallel to the direction of the ray of light 42 then the color of display approaches to the white. Namely, the visible property of the conventional liquid crystal display largely depends on the direction of view. For those reasons, the conventional liquid crystal display has a relatively narrow angle of visibility.
In order to settle the above problem, another conventional liquid crystal display illustrated in FIG. 3 was proposed, which utilizes a spray distortion of liquid crystal molecules 2 aligned between a common electrode 5 provided on a substrate 3 and a pixel electrode provided on a substrate 4. This conventional liquid crystal display is disclosed in the Japanese laid-open patent application No. 4-149410. FIG. 3 illustrates the liquid crystal molecules 2 when no voltage is applied thereto. The liquid crystal molecule 2 adjacent to the common electrode 5 on the upper substrate 3 differs in pretilted angle from the liquid crystal molecule 2 adjacent to the pixel electrode 6 on the bottom substrate 4. For example, the liquid crystal molecule 2 adjacent to the common electrode 5 on the upper substrate 3 is directed to a right-up direction, whilst the liquid crystal molecule 2 adjacent to the pixel electrode 6 on the bottom substrate 4 is directed to a left-up direction. The liquid crystal molecule 2 adjacent to the common electrode 5 on the upper substrate 3 is mismatched in pretilted angle to the liquid crystal molecule 2 adjacent to the pixel electrode 6 on the bottom substrate 4. In this case, the liquid crystal molecules 2 aligned between the top and bottom substrates 3 and 4 have a spray distortion. The liquid crystal molecule 2 positioned at an intermediate position between the top and bottom substrates 3 and 4 is directed almost parallel to the surfaces of the top and bottom substrates 3 and 4.
When a voltage is applied to the liquid crystal molecules 2, the liquid crystal molecules 2 particularly positioned apart from the top and bottom substrates 3 and 4 show change of directions but only in vertical component as illustrated in FIG. 4. The common electrode 5 differs in area from the pixel electrode 6. Since the area of the pixel electrode 6 is smaller than the common electrode 5, a slight field concentration appears in the vicinity of the pixel electrode 6. Namely, the field flux shows a slight convergence in the vicinity of the pixel electrode 6. In a right half domain 15, the liquid crystal molecules apart from the pixel electrode 6 on the bottom substrate 4 are directed to a right-up direction, whilst in a left half domain 14, the liquid crystal molecules apart from the pixel electrode 6 on the bottom substrate 4 are directed to a left-up direction. A ray of light 12-1 is directed to have a small angle to the right-up direction of the liquid crystal molecules in the right half domain 15, but to have a large angle to the left-up direction of the liquid crystal molecules in the left half domain 14. For those reasons, the ray of light 12-1 has a large transmissivity to the liquid crystal molecules in the right half domain 15, but a small transmissivity to the liquid crystal molecules in the left half domain 14. By contrast, a ray of light 12-2 is directed to have a large angle to the right-up direction of the liquid crystal molecules in the right half domain 15, but to have a small angle to the left-up direction of the liquid crystal molecules in the left half domain 14. For those reasons, the ray of light 12-2 has a small transmissivity to the liquid crystal molecules in the right half domain 15, but a large transmissivity to the liquid crystal molecules in the left half domain 14. This means that the right half domain 15 has a different optical property from that of the left half domain 14. The optical property is symmetrical between the right and left half domains 15 and 14. This may prevent any inversion of the black-white colors and whiteness.
FIG. 5 illustrates still another conventional liquid crystal display similar to the above display but differs in providing an opening 7 in a common electrode 5, wherein a voltage is applied to the liquid crystal. This display is disclosed in the Japanese laid-open patent application No. 6-43461. Electric field applied is not uniform due to the opening 7 of the common electrode 5 and a difference in area between the common electrode 5 and the pixel electrode 6. The common electrode 5 differs in area from the pixel electrode 6. Since the area of the pixel electrode 6 is smaller than the common electrode 5, a slight field concentration appears in the vicinity of the pixel electrode 6. Namely, the field flux shows a slight convergence in the vicinity of the pixel electrode 6. In a right half domain 15, the liquid crystal molecules apart from the pixel electrode 6 on the bottom substrate 4 are directed to a right-up direction, whilst in a left half domain 14, the liquid crystal molecules apart from the pixel electrode 6 on the bottom substrate 4 are directed to a left-up direction. A ray of light 12-1 is directed to have a small angle to the right-up direction of the liquid crystal molecules in the right half domain 15, but to have a large angle to the left-up direction of the liquid crystal molecules in the left half domain 14. For those reasons, the ray of light 12-1 has a large transmissivity to the liquid crystal molecules in the right half domain 15, but a small transmissivity to the liquid crystal molecules in the left half domain 14. By contrast, a ray of light 12-2 is directed to have a large angle to the right-up direction of the liquid crystal molecules in the right half domain 15, but to have a small angle to the left-up direction of the liquid crystal molecules in the left half domain 14. For those reasons, the ray of light 12-2 has a small transmissivity to the liquid crystal molecules in the right half domain 15, but a large transmissivity to the liquid crystal molecules in the left half domain 14. Those mean that the right half domain 15 has a different optical property from that of the left half domain 14. The optical property is symmetrical between the right and left half domains 15 and 14. This may prevent any inversion of the black-white colors and whiteness.
The above second and third liquid crystal displays illustrated in FIGS. 3-5 have the following problems. The liquid crystal molecules 2 positioned at a boundary between the right and left half domains 14 and 15 are kept to have a parallel direction to the surfaces of the top and bottom substrates 3 and 4 even when a voltage is applied to the liquid crystal. For this reason, the permissivity of the ray of light transmitting through the boundary between the right half and left half domains is lower than that of the ray of light transmitting through the right half and left half domains. Namely, the probability of scattering of the ray of light transmitting through the boundary between the right half and left half domains is higher than that of the ray of light transmitting through the right half and left half domains. As a result, the boundary between the right half and left half domains still remains somewhat white even the voltage is applied to the liquid crystal. The color of the boundary between the right half and left half domains does not become dark or black. Namely, the ray of light having transmitted through the boundary between the right half and left half domains is relatively white. This reduces a contrast between the white mode in no voltage application and the black mode in a voltage application.
In order to settle the above problems with whiteness, it was proposed to provide an optical shielding layer at the boundary between the right half and left half domains whereby the ray of the whiteness light having transmitted through the boundary between the right half and left half domains may be shielded by the optical shielding layer. The optical shielding layer may ideally prevent the whiteness of the display in the black mode when the voltage is applied to the liquid crystal.
Actually, the above liquid crystal display provided with the optical shielding layer at the boundary between the right half and left half domains is, however, has a problem with movement of the boundary between the first and second domains when a voltage is applied to the liquid crystal.
In FIG. 6, domains X are positioned outside domains Z which surround a domain Y positioned at a center. The electric field in the domains Z is uniform and just perpendicular to the surfaces of the top and bottom substrates 3 and 4, but in the domains X and Y the electric field is not uniform. In the domains X and Y, the liquid crystal molecules adjacent to the glass substrates 23 and 33 are directed in a direction H or L toward which the liquid crystal molecules are raised by the pretilted angle. Both the directions H and L are defined by rubbing as illustrated in FIG. 7. By contrast, in the domains Z, the liquid crystal molecules adjacent to the glass substrates 23 and 33 are, in random, directed in the directions H and L. Namely, there are two different domains H and L within which the liquid crystal molecules adjacent to the substrates 23 and 33 are raised by the pretilted angle toward the different directions H and L respectively. The boundary between the domains H and L are changed from immediately after the voltage is applied to the liquid crystal. FIG. 8 illustrates change of the boundary between the domains H and L from immediately after the voltage is applied to the liquid crystal. When the voltage application commenced, first liquid crystal molecules adjacent to the edge of the electrodes or in the peripheral region of the electrode do respond so that the liquid crystal molecules in the right bottom region and in the left top region are directed to the directions H and L respectively. Namely, the peripheral regions of the electrode in the right bottom region and in the left top region become the domain H and the domain L respectively, immediately after the voltage application commenced. On the other hand, at this time, the center regions except for the peripheral region of the electrode have the two domains H and L. Namely, the two domains H and L still coexist in the center regions except for the peripheral region of the electrode. With time, a region, where the coexistence of the two domains H and L, becomes narrow and is converged to a center position of the right bottom region and the left top region and then the right bottom region is entirely shared by the domain H while the left top region is entirely shared by the domain L. In the above processes, the boundary between the domains H and L is moved. Further, a time when the domain L disappeared in the left top region may differ from a time when the domain H disappeared. As well illustrated in FIG. 8, the boundary between the domains H and L are changed over positions after the voltage application to the liquid crystal commenced. Further, as described with reference to FIGS. 4 and 5, the boundary between the two different domains H and L has the problem with the whiteness of the ray of light having transmitted through the boundary and therefore it is necessary to shield the ray of such white light by providing a shielding layer in order to obtain a black color of the display in the black mode when a voltage is applied. Actually, however, the boundary between the two different domains H and L moves after the voltage application commenced, whilst the shielding layer is of course fixed. The movement of the boundary between the two different domains H and L does make it impossible to shield, by a shielding layer fixed, the ray of whiteness light having transmitted through the boundary. Further, it takes not less than 1 sec. until the domains H and L are stabilized and the boundary between them is fixed. During this, the boundary between the domains H and L are kept to move and draw a white line as a residual image on the display. The above phenomenon appears on the liquid crystal display illustrated in FIGS. 3 and 4.
If a rectangular-shaped opening is provided to extend parallel to one side of the electrode, first a boundary between the two different domains extends on or along a diagonal line as illustrated in FIG. 9. With time the boundary between the two different domains moves to approaches to a longitudinal center line of the rectangular-shaped opening. In this case, the boundary between the two different domains moves after the voltage application commenced, whilst the shielding layer is of course fixed. The movement of the boundary between the two different domains does make it impossible to shield, by a shielding layer fixed, the ray of whiteness light having transmitted through the boundary. Further, it takes 1 sec. in the order until the domains are stabilized and the boundary between them is fixed. During this, the boundary between the domains are kept to move and draw a white line as a residual image on the display.
Further, the Japanese laid-open patent application No. 63-106624 reduces a view angle dependency of the liquid crystal display by changing the directions of rubbing. As illustrated in FIG. 7, there are formed two different domains H and L on each of the substrates 23 and 33 by changing the directions of rubbing. As already described, the liquid crystal molecules adjacent to the substrate having already been subjected to rubbing are raised by a pretilted angle in the direction of rubbing. For this reason, the liquid crystals adjacent to the substrate having already been subjected to rubbing in different two directions as illustrated in FIG. 7 are raised by a pretilted angle in the different two directions of rubbing. The optical property of the display is symmetrical whereby the dependency of the visible property on the view angle is reduced.
Furthermore, the Japanese laid-open patent application No. 5-173135 forms four different domains within each pixel by moving one substrate in relation to another substrate so as to obtain symmetrical optical property as illustrated in FIG. 10.
The above conventional liquid crystal displays disclosed in the Japanese laid-open patent applications Nos. 63-106624 and 5-173135 have a disadvantage in needing to carry out rubbing in different directions for very small areas in each pixel. This requires an additional resist process and rubbing processes plural times. Further, it is required to carry out an alignment between top and bottom substrates at a high accuracy. Those result in increase of manufacturing cost of the liquid crystal display.
The Japanese laid-open patent application No. 6-194655 uses a chiral agent to cause a twist of 90 degrees of the liquid crystal molecules aligned between two substrates with orientation films subjected to no rubbing treatment as illustrated in FIG. 11. No rubbing treatment to the orientation films results in a week orientation force of liquid crystal molecules. The week orientation force of liquid crystal molecules allows a residual flow pattern formed in injection of the liquid crystal. The residual flow pattern of the liquid crystal prevents formation of the required orientations of the liquid crystal. In order to settle this problem, the substrates are subjected to a heat treatment so that liquid crystal is injected in isotropic phase and then cooled down to the room temperature. As a result, orientation vectors of the liquid crystal molecules continuously vary in direction over positions and the optical property of the display is symmetrical whereby the dependency of the visible property on the view angle is reduced.
Further, the Japanese laid-open patent application No. 7-92466 discloses use of a chiral agent and an orientation film of a pretilted angle of 0 degree in order to reduce a view angle dependency of the liquid crystal display by changing the directions of rubbing without conducting plural rubbing treatments as illustrated in FIGS. 12A and 12B.
The liquid crystal displays illustrated in FIGS. 7, 11 and 12A and 12B have a problem with the whiteness of the ray of light having transmitted through a boundary between different domains in which liquid crystal molecules have the same twist direction but different tilted-up directions as described above.
It was, as described above, proposed to provide an optical shielding layer for shielding the ray of light having transmitted through a boundary between different domains. However, the boundary between the different domains moves after the voltage application commenced until the different domains are stabilized as described with reference to FIG. 8.
Moreover, the optical shielding layer reduces an area of the screen of display whereby brightness of the screen is reduced.
Still more the Japanese laid-open patent application No. 6-242248 forms two different domains in which the liquid crystal molecules have different twist directions as illustrated in FIG. 13. Rubbing is made so that the liquid crystal molecules aligned between the substrates have a twist angle not less than 80 degrees but less than 90 degrees. The domains differs t from each other in rubbing direction so as to form different two domains in which the twist directions are different. As a result, the liquid crystal molecules adjacent to the substrate differ in tilted-up direction by 90 degrees between the different two domains. The optical property of the display is symmetrical whereby the dependency of the visible property on the view angle is reduced.
The above liquid crystal display has a problem with reduction in contrast between black and white colors. The liquid crystal molecules aligned between the substrates have a twist angle less than 90 degrees. This twist angle smaller than 90 degrees reduces the contrast between black and white colors.
Still further, the Japanese laid-open patent application No. 62-210423 provides an optical compensation plate having negative refractive index anisotropy between a liquid crystal cell and a polarizing plate. The optical compensation plate having anisotropic negative refractive index compensates double refraction due to a positive refractive index anisotropy of possessed by the liquid crystal molecules. In addition, it is disclosed in the Japanese laid-open patent applications Nos. 6-214116 and 7-20456 to use an optical compensation plate having an optical axis which is tilted from a vertical axis of the plate.
Actually, however, the use of the optical compensation plate improves but insufficiently the asymmetry of the optical property of the liquid crystal display.
It was therefore required to develop a novel liquid crystal display free from the above problems and disadvantages until the present invention was made, which will be described hereafter.